What's currently the best, most "open" phone on which to write custom software?

Years ago, I made the mistake of buying an LG VX6000. It's a decent phone, but poor target platform for programming.

First, I could only write software in C++, which had to link to a Windows-only development environment. Installing my custom software on the phone required that I first mail it to LG so that they could "unlock" it. And if I wanted to publish the software, I had to pay Verizon $500 for the privilege. And of course, if I wanted to add anyone else's software, I'd have to pay serious $$$.

I understand these are just corporate gimmicks for filtering out the "hackers" who just want to piss people off, but as a programmer I'd like to have a phone that I can freely modify, and install the program's I want without have to get the approval of <insert huge corporation>.

Google's Android based phones sound pretty close to what I'd like, but since none of these actually exist yet, what's the next best thing?

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